In Cat.IIIa operations (747) the "alert height" generally occurs between 30 and 50 feet AGL... the approach is flown on autopilot with autoflare capability. The aircraft we so operate are technically capable of Cat.IIIb operations (again autopilot) but the 747 is not approved to these minimums.
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There are two Cat.II minimums, (I call them the 150 and the 100 Cat.II DH). One requires full equipment working, the other permits approaches to the higher minimums with some equipment inoperative...
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Happy contrails
(s) Skipper